The art of railway brakes includes two methods of retarding a railroad vehicle. One method is to provide brake shoes which may be pressed against the wheels of the railroad vehicle, to provide a friction force which retards the wheels. A second method is to attach a disc to a wheel or to an axle of the vehicle, and press brake shoes against the disc, thereby providing a friction force which retards the disc and hence retards the wheels. In the first of these methods, heat is absorbed by the thermal mass of the wheels, and then dissipated to the environment by conduction, convection, and radiation from the wheels. In the second of these methods, heat is absorbed by the brake discs, and then dissipated to the environment by conduction, convection, and radiation from the discs.
In both cases, the amount of energy which can be absorbed is limited by the temperatures generated, since high temperatures may damage the brake shoes, or cause thermal stresses which cause cracking of the wheels or brake discs. In some systems, the two methods are combined so that some of the heat is absorbed by the wheels, and some is absorbed by the discs. By combining the two methods, more heat can be absorbed than can be absorbed by either method separately.
In some systems employing disc brakes, brake shoes are applied to the rims of the brake discs. In others, brake shoes are applied to the faces of the discs. Application to the rim has an advantage over application to the face because the radius at which the friction force is generated is greater when it is applied to the rim than when it is when it is applied to the face. Hence, the retarding torque exerted on the brake disc is greater when the shoe is applied with a given force to the rim than when the shoe is applied with the same force to the face.
For a railway vehicle, it is particularly important to apply the brake shoe at as great a radius as possible because the radius of the disc is limited by the required track clearance. In the United States, this is 2.75 inches, so the radius of the brake disc must be at least 2.75 inches less than the radius of the wheel tread.
It is generally desirable for a brake system to provide a system for applying pressure to the brake shoe which provides mechanical advantage to amplify the total normal force between the brake shoe and the surface being retarded. For a brake shoe applied to the rim of a brake disc, prior attempts to accomplish this have been made by having the braking surface of the brake shoe have the form of a wedge, which is applied to a groove on the rim of the brake disc. With this configuration, the total normal force between the brake shoe and the brake disc is greater than the inward radial force applied to the shoe. An example of this is provided by U.S. Pat. No. 2,422,004.
Such configurations, however, are badly affected by tolerance in the position of the brake shoe relative to the brake disc in the direction of the axis of the brake disc. Variations in the exact relative axial positions of the brake shoe and the brake disc cause the apex of the wedge and the bottom of the groove on the brake disc to become rounded as wear occurs. Hence, some surface is produced at the apex of the wedge and the bottom of the groove which is not inclined relative to the radius. A portion of the inward radial force applied to the brake shoe is then borne by this surface, and for this portion of the shoe force, the normal force is not amplified. Hence, after some wear occurs, such a brake looses efficiency.
Furthermore with such a configuration, wear material from the brake pad may become trapped between the brake shoe and the brake disc and interfere with proper braking.
Heat generated at the friction surface of the brake disc must flow through the material of the brake disc until it reaches a surface which is cooled by the air, and/or cooled by radiation. Allowable braking forces must be limited to prevent excessive temperatures which would destroy the pad material, or cause cracking of the brake disc.